There is a need for a technique to form a consolidated product of enhanced densification from rapidly-solidified iron-based powders wherein the fine microstructures present in such powders are retained. In the past, it has been assumed that this requires low temperatures (e.g. less than 800.degree. C.) to avoid coarsening of the microstructure. However, correspondingly high pressures or stresses would be required to form a product of high density. The use of extremely high pressures is a limiting factor in the manufacture of consolidated powder products. Thus, there is a need for reducing the pressure required at low temperatures for manufacture of a consolidated product of high densification.
One approach to densification of iron powders is suggested by S. Kohara, in "Effect of Repeated Allotropic Transformation on Sintering of Iron Powder", Metall. Trans., 1976, Vol. 7, p. 1239. Pure iron powder was utilized which has a transformation temperature of 910.degree. C. Extremely small stresses of 10 psi (approximately 0.1 MPa) were applied. The limited enhancement of densification was attributed to the occurrence of transformation superplasticity.
Another approach was suggested by Y. Oshida, in "An Application of Superplasticity to Powder Metallurgy", J. Jpn. Soc. Powder and Powder Metall., 1975, Vol. 22, p. 147. There, the densification of cast iron powders by multiple thermal cycling through the A.sub.1 transformation temperature (727.degree. C. for Fe-C alloys) under small applied stresses of 70 to 210 psi (0.5 to 1.5 MPa) was investigated. The enhancement of densification was attributed to transformation superplasticity. However, experiments have shown that under the transformation cycling conditions employed by Oshida, significant densification would only be expected if stresses substantially above 1.5 MPa had been applied.